The present invention relates generally to systems, kits and methods for joint prosthesis installation tools. The present invention includes jigs and methods for installing a stemless ball component to a prepared femoral head along an axis defined by the prepared femoral head and jigs for installing a sleeve component for adapting a ball component to a prepared femoral head along an axis defined by the prepared femoral head.
Artificial joint prostheses are widely used today, restoring joint mobility to patients affected by a variety of conditions, including degeneration of the joint and bone structure. Typically, the failed bone structure is replaced with an orthopedic implant that mimics, as closely as possible, the structure of the natural bone and performs its functions. The satisfactory performance of these implants can be affected not only by the design of the component itself, but also by the surgical positioning of the implanted component and the long-term fixation of the implant. Improper placement or positioning of the implant can adversely affect the goal of satisfactorily restoring the clinical bio-mechanics of the joint as well as impairing adequate fixation of the component when implanted.
Orthopedic implants are constructed from materials that are stable in biological environments and withstand physical stress with minimal or controlled deformation. Such materials must possess strength, resistance to corrosion, biocompatibility, and good wear properties. Also, the implants include various interacting parts, which undergo repeated long-term physical stress inside the body.
For these reasons, among others, the bone/implant interface and the connection between various parts of the implant must be durable and resistant to breakdown. This is especially important since installation of an orthopedic implant often involves an extensive and difficult medical procedure, and therefore replacement or revision of the installed implant is typically difficult and traumatic.
The requirements for the useful life of the implant continue to grow with the increase in human life expectancy. Also, as implants improve, younger patients are considered as implant candidates. It is therefore desirable to develop implants that, while durable in their own right, minimize the difficulty of replacement
The strength and longevity of implants in large part depend on the bone/implant interface. Various methods of connection are known in the art. For example, a hip joint is a ball-in-socket joint, and includes a rounded femoral head and a cup-like socket (acetabular cup) located in the pelvis. The surfaces of the rounded femoral head and the acetabular cup continually abrade each other as a person walks. The abrasion, along with normal loading, creates stress on the hip joint and adjacent bones. If the femoral head or the acetabular cup is replaced with an implant, this stress must be well tolerated by the implant's bearing surfaces to prevent implant failure.
Depending on the type of bone, the location of the bone within the body and individual characteristics, bone has a wide variation in mechanical characteristics. Bone is generally categorized as trabecular or cancellous bone, which is porous and has an open cancellated structure, and cortical bone, which is dense. Considering the femoral bone of the hip joint, FIG. 1 shows the proximal portion of a femur 1 with the upper portion of the shaft 3, a neck 5 and a head 7. A shaft axis A-A is aligned with the shaft 3 and a head axis B-B is generally aligned with the neck 5. The shaft 3 is primarily composed of cortical bone while the neck 5 and head 7 are primarily composed of trabecular bone with cortical bone at the surface.
Implantable joint prostheses have long been used to provide an artificial hip. When the prosthesis is situated in this position, significant forces such as axial, bending, and rotational forces are imparted to the device. Conventional total hip replacements use an intramedullary stem as part of the femoral prosthesis. The stem passes into the marrow cavity of the femoral shaft. These stem type prostheses are very successful but when they fail the stem can create considerable damage inside the bone. The implant can move about inside the bone causing the intramedullary cavity to be damaged. Because a stiff stem transmits the forces more directly into the femoral shaft, such implants have the further disadvantage that they can weaken the surrounding bone nearer to the hip joint due to stress shielding.
Early designs of femoral prostheses for artificial hips relied primarily on cemented fixation. These cements, such as polymethylmethacrylate, are used to anchor the component within the medullary canal by acting as a grouting agent between the component and the endosteal (inner) surface of the bone. While this method of fixation by cement provides immediate fixation and resistance to the forces encountered, and allows the surgeon to effectively position the device before the cement sets, it is not without problems. Over time, the mechanical properties and the adhesive properties of the bone cement degrade; eventually the forces may overcome the cement and cause the components to become loose due to a failure at the cement/bone or cement/stem interface. Alternative approaches to address the issue of cement failure include both biological ingrowth and press-fit type stems.
Stems designed for biological ingrowth typically rely on the bone itself to grow into a specially prepared surface of the component, resulting in firmly anchoring the implant within the medullary canal. A shortfall of this approach is that, in contrast to components that utilize cement fixation, surfaces designed for biological ingrowth do not provide for immediate fixation because it takes time for the bone to grow into the specially prepared surface. Press-fit stems precisely engineered to fit within a surgically prepared medullary canal may or may not have specially prepared surfaces and typically rely on an interference fit of some portion of the component within the medullary canal of the bone to achieve stable fixation.
The need often arises to replace at least a portion of a hip implant. Prior art designs often require the entire implant to be replaced even if only a portion of the implant fails. Similarly, the entire implant may have to be replaced if the implant is intact but certain conditions surrounding the implant have changed. This is often due to the implant suffering from a decrease in support from the adjacent bone due to stress shielding or other negative effects of the implant on surrounding bone.
Surgeons have sought a more conservative device than an implant using an intramedullary stem as part of the femoral prosthesis. There have been a number of attempts at implants using short stems or femoral caps without stems and requiring less extensive surgery. This type of prosthesis is generally known as a hip resurfacing prosthesis. In the mid-1940's, Judet in France designed a prosthesis whereby the majority of the femoral head was removed and a replacement device was fitted with a peg or nail which passed a short way down the femoral neck. Small movement of the device against the bone caused friction of the bone and the bending loads on the peg often caused them to break out underneath the bony femoral neck. In the mid-1970's, double cup type arthroplasty was tried. There were several designs: Wagner in Germany, an Italian Group, Imperial College London and the Tharies design from Amstutz in California.
These all removed a fair proportion of the femoral bearing surface by turning it down to a cylindrical form or hemispherical form. A metal shell was then fixed with bone cement on the remaining bony peg. The acetabular cup was conventional. Unlike normal total hips, however, which have standard femoral head sizes in the range of 22-32 mm, these double cup arthroplasties have large bearing surface diameters closer to the original hip, typically in a range from 40-60 mm. These latter double cup designs commonly failed either by a crack progressing around the bone cement between the prosthetic femoral shell and the bone or by a fracture of the bone across from one side of the prosthetic femoral component rim to the other.
Current approaches to femoral head resurfacing can be traced back to Amstutz in U.S. Pat. No. 4,123,806. In the '806 patent, a hemispherical cap is cemented to a prepared femoral head while preserving a substantial portion of the femoral head. In U.S. Pat. No. 6,156,069, Amstutz shows a femoral head resurfacing implant having a stem. A similar femoral head resurfacing technique having a stem called Birmingham Hip Resurfacing has been developed by McMinn in the United Kingdom.
These stem-type femoral head resurfacing prostheses consist of a bearing cap provided with a central pin that guides the prosthesis during the insertion. The guiding is important because it ensures that the prosthesis will be seating at the appropriate orientation planned by the surgeon with regard to the bone. A consequence of the misalignment of the prosthesis is a sub-optimal load transfer to the bone that can lead to the failure of the prosthetic joint. Similar to the problems with the prostheses having a stem extending into the femoral shaft, a stem-type resurfacing prostheses requires the surgeon to remove enough bone in the neck of the femur so that it can host the pin of the prosthesis and the stem can contribute to stress shielding. Therefore the stem-type prosthesis is not as bone preserving as a stemless prosthesis, either in the short term or long term.
Notwithstanding the problem of guiding a stemless prosthesis, stemless approaches have been advocated and continue to be developed. A modular approach to a stemless femoral hip resurfacing is shown in U.S. Pat. No. 4,846,841 to Oh. In this approach, a frustro-conical cap is press-fit to a prepared femoral head. A ball component is then attached to and retained by the cap using a Morse taper fit. A similar approach is shown in U.S. Pat. No. 5,258,033 to Lawes and Ling, which shows a ball component cemented either directly to a prepared head or, additionally, retained by a press-fit with a frustro-conical cap. A contemporary approach to stemless femoral head resurfacing is found in co-owned U.S. patent application Ser. No. 11/478,870 entitled, Femoral Head Resurfacing.
All of these more modern hip resurfacing approaches require that the femoral head be prepared to provide a properly oriented and shaped bone interface for the implant by shaping the head. The outer prepared bone interface with the implant is symmetrical around a prepared head axis B-B passing through and established with reference to the central region of the femoral neck and is typically cylindrical or conical but may be a more complex tapering solid of revolution. The proximal portion of the prepared head can be a flat surface, tapered, domed, chamfered, or any combination of these features and is usually performed as a separate resection. If a stem is used, it may be cylindrical, conical or a more complex tapering solid of revolution and is typically short compared to a conventional intramedullary stem. The portion of the bone that hosts the prosthesis must be shaped so that it matches the shape of the prosthesis. The size and shape of the bone may fit exactly the shape and size of the prosthesis or may provide room for cementing to take place or have an excess of bone in a region to allow press-fit fixation, depending on the preferred fixation method.
Because the desired bone shape of the outer implant interface is symmetrical around an axis, a guide wire or pin introduced into the femoral head is typically used to establish the tooling landmark or datum that establishes and defines the prepared head axis B-B. The guide pin provides an axis of revolution for the various measuring and cutting tools used in the preparation process as shown in FIG. 4 and discussed later. When introduced into the femoral head, the guide wire creates a corresponding bore that also provides a datum for the axis B-B in the absence of the guide wire. Based on pre-operative planning, the surgeon initially places the guide wire, either freehand or using measurement and guidance tools based on various anatomical reference points on the femur. In order to place the pin, the pin is driven or inserted in the proximal surface of the femoral head directed toward the greater trochanter and approximately down the mid-lateral axis of the femoral neck. A gauge having an extended stylus that allows measurement of the position of the pin with respect to the neck is then typically used to make a preliminary check of the pin position. By revolving the gauge, the surgeon can evaluate the position of the pin to ensure that the femoral neck will not be undercut when the cutting tool is revolved around the pin. If the surgeon is satisfied that the pin position meets these criteria, the guide wire is used as the axis of revolution for the shaping cutter or reamer to prepare the head to receive the implant.
For a traditional stem-type resurfacing prosthesis, the stem cavity is used to guide the stem and prosthesis into position. Thus, a stem-type resurfacing prosthesis has provided an acceptable method of guiding the prosthesis with respect to the femoral head axis because the stem cavity approximates the head axis datums and the stem serves as tooling to guide the prosthesis along an axis approximating the femoral head axis during installation by engaging the stem cavity.
For a stemless prosthesis, the stem and stem cavity do not exist and the stem cannot serve as tooling to guide the prosthesis. It is desirable to have tooling and a surgical method for use with a stemless prosthesis that allows implanting the prosthesis along the femoral head axis with at least the same accuracy achieved by stem-type resurfacing prosthesis.